Posted by KurtP (A) on October 10, 2007 at 05:43:11

Thought a new thread might be recommended at this point.

6n6p series feed with LL1689AM/PP. Heaters are currently using regulated DC voltage from a home brew choke input supply to LM1086 regulators.

Also working on a second project, a 5V DC supply to feed 300B filaments. Very future project, likely a Karna amp. Got the prototype completed last night, and started poking around with the scope, to make sure my supply is quiet. Found significant RF hash on the output due to rectifier switching/choke ringing, exactly like what is described in Morgan Jones' book. I'll be posting at DIY for some assistance on this front.

However, this made me poke around a little bit in the series feed amp, in which I discovered the same problem. RF has always been my enemy, and I'm not very good at taming it. If I get any success with the 5V 300B supply, I'll use the same solution here, but for now, I'm considering the possibility of just going with AC heating on the series feed.

So my question is, what do you use, AC or DC for the 6n6p? Being this is my first stage past the RAKK DAC, I want to ensure hum-free operation. I would use shielded twisted pair for the heater lines, of course, and elevate them by 30V.

Thanks.

• RE: AC/DC heating with series feed - KevinC 07:45:25 10/10/07 (1)
  In Reply to: RE: AC/DC heating with series feed posted by KurtP on October 10, 2007 at 05:43:11

  AC heating has proven to be adequate for 6N6Ps in the line stage. Broadband noise dominates the output noise of the line stage, so hum from the filament supply is a non-issue.
  
  I also supply the 300B filaments in the Art Audio Vivo with AC. I use a 5 ohm wirewound pot flanked on either side with a 20 ohm resistor to provide hum balance and a cathode resistor connection point. The hum from the Vivo in my Ariels is so small that I can't hear it until my ear is 6" from the mid-woofer cones. It's a big deal to make a quiet DC supply for high current indirectly heated filaments and AC works very well.
  
  Kevin Carter
  K&K Audio
  www.kandkaudio.com
  
  

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  • RE: AC/DC heating with series feed - KurtP 10:15:59 10/10/07 (0)
    In Reply to: RE: AC/DC heating with series feed posted by KevinC on October 10, 2007 at 07:45:25

    "It's a big deal to make a quiet DC supply for high current indirectly heated filaments and AC works very well."
    
    You aren't kidding. I continue to be plagued by RF in my DC supplies, built with all the precautions.
    
    My suspicion is this may be contributing to my listening fatigue. I'll try AC and see what happens.
    
    Little more difficult to check the B+ noise with a lab scope, but that might be worth a check. Any ideas on how to best to that?
    
    

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• RE: AC/DC heating with series feed - AndyN 06:08:02 10/10/07 (1)
  In Reply to: RE: AC/DC heating with series feed posted by KurtP on October 10, 2007 at 05:43:11

  I guess youhave a choke input filter and the choke is ringing. You may to try either Morgans suggestion of 100nF caps to ground either side of the choke or the old fashioned 10nF and 1 to 10K across the choke. In my experience this fixes the ringing.
  
  Have you tried getting rid of the regulator for the DC heaters? Replace it by a suitable wattage resistor - you may well be surprised and discover you prefer the sound. All regulators have an indictive output (their impednace rises with frequency). You could also try following the regulator with an RC network - say 3R3 and 100nF - this may well yield similar results.
  
  Personally I use AC heaters in my power amp' but DC heaters in the pre-amp.
  
  Hope this helps.
  
  Andy
  
  

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  • RE: AC/DC heating with series feed - KurtP 07:14:46 10/10/07 (0)
    In Reply to: RE: AC/DC heating with series feed posted by AndyN on October 10, 2007 at 06:08:02

    Yes, I have choke ringing, no question about it. Secondary of transformer is ugly, looks just like the book. I've tried Morgan Jones' method, to little avail. I also notice it takes a lot of capacitance (4 uF) across the choke to significantly change the ringing waveform. And yes, I am using the RC on the output of the regulator. Thought I was taking all the precautions, but so far the results have not been stellar.
    
    As far as removing the regulator, I would say if I go DC, the regulator stays. I see little benefit in not regulating, after going through all the trouble of rectifying. Besides, my issue won't change if I remove the regulator; I'll still have RF hash.
    
    The next few days I'll try to clean this up, but if I end up with no hum using AC heaters, why not? Certainly makes things easier.
    
    

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